U.S. Special Envoy to Sudan and South Sudan Donald Booth on Monday urged South Sudan not to slide back into violence after clashes rocked Juba overnight, but refused to call the unrest in the capital an attempted coup.

"The situation remains unclear as to what exactly sparked the violence... We are not confirming a coup attempt yet," Booth told VOA News in an interview.

"We've been reaching out to numerous parties in Juba as well as others in the region to put together a picture of what exactly happened," Booth said, adding that "the new country cannot afford a slide back into violence."

President Salva Kiir told a news conference earlier Monday that a group of soldiers allied to former Vice President Riek Machar, who was sacked in July by Kiir as part of a complete cabinet reshuffle, had launched an attack on army headquarters in Juba, plunging the capital into a night of violence.

His government had regained control as of Monday morning, Kiir said, even as witnesses reported hearing continuing gunfire and explosions in Juba.

The U.S. embassy issued a statement calling for all sides to settle their differences through "peaceful political means." It warned U.S. citizens in South Sudan to remain indoors until calm has been restored.

Booth told VOA that embassy personnel in Juba are themselves sheltering in place "until they feel it's safe to move around the city."

Juba is under a dusk to dawn curfew until further notice and the international airport was closed in response to the violence.

Although VOA reporters in other South Sudanese states said the situation there was calm, State Department spokeswoman Marie Harf expressed concern that the violence could spread to other countries in the volatile region, and said Washington was continuing to monitor the situation.

The United Nations Mission in South Sudan (UNMISS) said in a statement that hundreds of civilians, many of them women and children, have sought shelter at the U.N. compound near Juba's international airport and at U.N. House in the Jebel Kujur area of the city.

In the statement, UNMISS also denied "any suggestion that the UN Mission is harboring any key political or military figures" as rumors circulated that Machar had sought refuge with the U.N. or with a foreign embassy.

Machar's whereabouts were unknown Monday, and VOA was unable to reach him or his spokesman for comment.

Since he was fired five months ago, Machar has been highly critical of Kiir, saying he has "dictatorial tendencies" and vowing to challenge him for the leadership of the ruling Sudan People's Liberation Movement (SPLM) party ahead of general elections in 2015.

Booth told VOA News that the United States has been "concerned for awhile about the rising tensions between different factions of the SPLM" and said he called during recent visits to South Sudan for all sides to resolve any disputes they might have through dialogue, not violence.